1. Field of the Invention
The invention relates to a colour display tube system comprising: a) an evacuated envelope having a neck, a cone and a display window, b) an electron gun in the neck, which gun has a beam-forming part for generating a central electron beam and two outer electron beams whose axes are co-planar, and an electrode system which in operation constitutes a main lens, c) a deflection unit for generating deflection fields for deflecting the electron beams in the horizontal and vertical directions and for scanning the display window by means of convergent beams.
2. Description of the Related Art
Colour display tube systems of the type described in the opening paragraph are commonly referred to as 3-in-line systems. Generally, the guns of these systems are constructed so that the outer beams converge upon leaving the gun, and comprise self-convergent deflection units which in operation generate non-uniform magnetic fields for horizontal and vertical deflection (particularly a barrel-shaped field for the vertical deflection and a pincushion-shaped field for the horizontal deflection) so that the three electron beams generated by the electron gun and focused on the display screen by the main lens converge throughout the display window.
However, these deflection fields cause the horizontal spot growth to increase by a given factor in the case of deflection, which factor may be more than two in 110.degree. colour display tube systems. This notably means that in a normal self-convergent system, in which the three guns are located in a horizontal plane, a circular central spot becomes narrower in the vertical direction and very elongate in the horizontal direction when scanning the screen. As a result a loss of resolution occurs in the horizontal direction and there is a risk of Moire problems owing to the spot becoming narrower vertically and the existence of horizontal dams in the shadow mask. The increasingly strict requirements imposed on the definition of the image, notably in high-resolution colour monitor tubes or when using colour display tubes for high-definition television with an aspect ratio of approximately 9:16 of the display window, require that at the ends of the horizontal axis the spot should be small in the horizontal direction.